Method and device for corrosion protection of electrochemical cells

Abstract: The invention relates to a method of lining surfaces subjected to corrosion in electrochemical cells, especially chlorine alkali cells, but also chlorate cells, e.g. in cell covers (2) to chlorine alkali cells. The cell cover (2) in question is provided with a prefabricated titanium lining (1) which covers all surfaces on the cell cover which during operation come into contact with the process liquids which are corrosive or are in any other way chemically aggressive. The titanium lining is prefabricated in dimensions corresponding to the measurements of the cell cover, and are mounted in situ at the plant by means of clamping joints. To protect the titanium in the lining from corrosion it is given an anodic potential via a resistance which is connected to a point having anodic potential. Exposed parts may also be protected by noble metal oxide or noble metal.

Description: The present invention relates to a process for covering or lining surfaces of electrochemical cells subjected to corrosion, especially chlorine-alkali cells, but also chlorate cells. It also relates to a device for achieving such protection. The state of the art Today, the most usual way of protecting parts of electrochemical cells subjected to corrosion is to line them with polymer materials. For chlorine cells various forms of hard rubber are the most common materials. For pipes, polyester reinforced with fibre glass is commonly used. These materials, however, wear out in the aggressive environment of the cells, and must be replaced by new linings after a certain time, about 10-15 years. It has also been shown that the polymer materials, when they are subjected to high chlorine activities, form highly chlorinated organic compounds such as hexachlorobenzene and various dioxines. These compounds are formed by the reaction between chlorine and the various compounds in the said construction material. Analyses of the environment in the vicinity of chlorine factories have shown increased levels of said environmentally burdening substances. Thus, there is a need for corrosion protection of a different type which does not generate such environmentally destructive substances. Titanium is one of the few materials known which is not reactive in the environment which exists in the anode chamber of a chlorine alkali cell. It would, therefor, be an excellent alternative for effective corrosion protection. There is today a technique which makes use of titanium for corrosion protection for cell covers of chlorine-alkali cells. According to this technique the cell cover in need of renovation is sent to a workshop where, by means of a special welding technique, a titanium cover is attached onto the steel of the cell cover. Since titanium cannot easily be welded onto steel, special welding methods are required, which cannot be carried out just anywhere. Before the titanium is welded on, the cell cover must be released from old hard rubber in order to obtain a clean steel surface, which is a time-consuming and, to a certain extent, an environmentally destructive procedure, due to, i.a. the use of pyrolysis to release the rubber. During this process mercury, for example, remaining on the rubber, as well as possibly existing dioxines may be released. Not even after cleaning the rubber very carefully before the pyrolysis may one be sure that all the environmentally dangerous materials have been removed. Another method used for removing rubber is the use of a low temperature, for example, by means of liquid nitrogen, etc. After the removal of the old, polluted rubber, the question of its deposition and the costs involved still remains. Because the cell cover must be welded on, one has difficulties, for example, in getting access to the welding joints to protect them with gas during welding, for example, in the process lead throughs on the cell cover. These welding joints are difficult, if not impossible, to control with regard to quality, i.e. whether it is leak-proof. Furthermore this is a very expensive technique. Summary of the Invention The purpose of the invention is to achieve a method of protecting against corrosion of, for example, cell covers of chlorine alkali cells, which eliminates the disadvantages of known techniques. This purpose is achieved by providing the cell cover in question with a prefabricated titanium lining which covers all surfaces on the cell cover which during operation come into contact with process liquids which are corrosive or are chemically aggressive in other ways. This process is defined in claim 1. By prefabricating the titanium lining or coating, which comprises a second aspect of the invention and is defined in claim 14, one achieves a number of advantages. One avoids i.a. heavy and expensive transport of cell covers to the workshop in another area, and the production of the lining or coating can more easily be controlled with regard to the tightness of welding joints, for example. Preferably the titanium metal is protected by giving it an anodic potential. In a preferred embodiment of the invention, especially exposed parts of the titanium lining are protected by a mixed oxide coating consisting of oxides of titanium and noble metals. The especially exposed parts are those areas which form small slits or columns, for example at the process lead throughs. Embodiments of the invention will now be described in more detail with reference to the attached drawings, in which Fig. 1 illustrates schematically a chlorine alkali cell having a rigid cover; Fig. 2 illustrates schematically a chlorine alkali cell having a floating cover; Fig. 3 illustrates a detail of the floating cover of Fig. 2, and more specifically how a cloth is braced in the cell; Fig. 4 illustrates schematically a prefabricated titanium lining according to the invention, as seen from above; Fig. 5 illustrates one type of process lead through where the titanium lining is braced in said lead through; Fig. 6 illustrates another type of process lead through where the lining is also braced; Fig. 7 illustrates a further method of bracing the titanium cover sheet or plate on the cell cover; Fig. 8a illustrates a method of bracing a fluoroplastic cloth on a cell; Fig. 8b illustrates an alternative method of bracing a fluoroplastic cloth on a cell; and Fig. 9 illustrates the principle of how an electric resistance is connected between the titanium lining and a point having anodic potential. After 5 - 7 years operation of the chlorine alkali cells of the type illustrated in Fig. 1 and 2, the hard rubber lining usually used on the cell covers for protection against corrosion will have been attacked because of the aggressive environment and no longer fulfil its function, and consequently must be replaced. As already mentioned, hitherto one has been forced to transport the cell covers to be renovated to special workshops for this purpose. Thereby the aged rubber has been removed and replaced with new material, or, in some exceptional cases, a titanium lining is welded onto the steel of the cell cover subsequent to the removal of the rubber. According to the invention a titanium lining 1 is prefabricated in accordance with the dimensions of the cell cover 2 (cf. Fig. 1 and 2), and is transported to the cell in the plant. The prefabricated titanium lining 1 (later described in detail) is mounted in situ at the chlorine alkali factory. Then first the existing rubber 3 in the process lead throughs 4 in the cell cover 2 is removed to create a tolerance for the pipes 5 arranged on the titanium plate. If desired, remaining rubber may be left on, which is a great advantage with the invention. By allowing the rubber to remain on, a compact encasement of the environmentally damaging polluted rubber is obtained since the titanium will surround it. Thus, there is no need to take care of the old rubber, which may be regarded as risk-factor waste. If one decides to remove the rubber, this need not be done as carefully as in conventional technique, i.e. if new rubber is applied or if the titanium lining has been welded on. The most suitable way to remove the rubber in the latter case is to cut/hew away after cooling, e.g. by means of liquid nitrogen, which makes the rubber glass-like and thus, easily worked. The thus prepared cell cover is placed upon the prefabricated titanium plate, where the pipes are passed through the process lead throughs. Thereby slits or columns are formed between the titanium and steel in the cover, which are filled with a suitable sealing compound. The demands on this sealing compound are only that it shall be resistant to the mediums which may be thought to exist in the air of the cell chamber and in residues of water and salt solution as well as giving such adhesion to both steel and titanium that leakage does not occur and the liquid collects on the upper side of the titanium, between steel and titanium. Should such leakage arise the steel will-corrode. Examples of suitable sealing compounds are various commercially available silicones as well as cements based on polymer, which are well known to a man skilled in the art. The prefabricated titanium lining 1 according to the invention consists of a titanium plate (cf. Fig. 4) having mainly the same size as the cell cover 2. This plate is provided with holes 6 which correspond to all the inlets or lead throughs 4 of said cell cover 2 for various process parts. The holes are punched or cut out by means of suitable cutting methods, e.g. laser or water cutting. For each hole a pipe 5 is welded (cf. Fig. 5 and 6) which will form the new lead through. Because one finalizes all the welding work before assembly of the lining, one has much greater possibilities of controlling the joints with regard to tightness, e.g. by means of x-ray or penetration with a dye. This is difficult to do in traditional lining techniques since one welds fast the lining directly onto the steel in the cover. The embodiment of the lead-through which is illustrated in Fig. 5 comprises that the pipe 5, welded on the titanium lining 1, is provided with an outer thread 7. A locking nut is screwed on the pipe thread 7 from above subsequent to the cover 2 being placed on the titanium plate 1. To fill the slit or column 9 which is formed between the protective sleeves 10 and the pipe 5, a sealing nut 11 of fluoroplastic e.g. of PFA, is furthermore screwed on, down the locking nut 8. The sealing nut 11 has therefor a conically chamfered lower part 12 which is rigidly wedged in the column 9. Fig. 6 illustrates an alternative embodiment of an inlet. Here the pipe 5 is not provided with a thread, but instead a sealing ring 13 of e.g. PFA is pressed down into the column 9 between pipe 8 and the sleeve 10 by means of a washer 14, which is pressed fixedly by a plate 15, of e.g. bakelite with the aid of pin bolts 16 and nuts 17. Also here the sealing ring 13 is conically chamfered at the lower end 12 to be wedged securely in the column 9. A spring tension plate 18 of the type Seeger ring is placed against the outer surface 19 of the pipe by means of a washer 20, e.g. of polymer material, between itself and the cell cover 2. The upper ridge of the sealing ring is formed into a flange 21. Above this flange the washer 14 is placed for equalizing forces, and upon this is arranged as mentioned, a plate 15 of e.g. bakelite, which is placed against the sealing ring. The nuts 17 are then screwed onto the bolts 16 thereby pressing the aggregate together. In both embodiments the column 9 which is formed between the outer surface 19 of the pipe 5 and the lead-through 4, 5 is filled out with the above mentioned sealing compound. The titanium plate 1 can be fixed to the cell cover 2 by means of various methods. One method, as described in connection with Fig. 5, is to provide the pipes 5, which run through the cover 2 in the process lead throughs 4, with an exterior thread 7 and thereafter bracing on the titanium plate 1 with a nut 17. One variation of this method is also illustrated in Fig. 7. Here the methods of Fig. 5 and Fig. 6 are combined by using a locking nut to hold the titanium lining in place, and a sealing ring of fluoroplastic. This gives a dependable unit which allows the anodes to move without putting a mechanical load on the welding joints, which may be the case when mounting the lining by means of welding according to the state of the art. To facilitate and speed up mounting, one may, as has been mentioned, use clamping rings of the type Seeger rings. If also these components, which are not in direct contact with the aggressive process medium are made of titanium, the risk of the circulating salt solution being polluted by the hydrogen gas developing components, such as vanadium, chromium, etc. is reduced, these being common alloy materials in many construction materials. The need of maintenance of these components is reduced as well since they stand up to the harsh environment which exists in an electrolyser hall in a chlorine factory. The anodes of the cell are mounted by passing their protective sleeves 10, from beneath, through the pipes 5 arranged on the titanium plate. The inner diameter of the pipes is selected so that a small slit or column 9 results to facilitate mounting. This slit must be sealed to prevent leakage of chlorine gas from the inner part of the cell to the surroundings. It has been found to be suitable to use fluoroplastic for the sealing, and it can, as mentioned above, be formed into a ring having conical geometry, which presses down the part between the inner surface and the anode's protective sleeve 10. The sealing 13 of fluoroplastic is kept in place by means of a spring tension plate 14 of titanium which is arranged on the upper side of the sealing 13. If the cell cover is of the type where the cover also has the function of supporting the anodes, the cover must be able to move up and down. This mobility is achieved by securing a fluoroplastic cloth 22 to the periphery 24 of the cover, and in the outside edges 23 of the cell box (cf. Fig. 2). The fluoroplastic cloth 22 may be secured at fastening points provided on the cell box. These must, however, be modified to allow the cloth to be mounted without the occurrence of leakage. In the embodiment illustrated in Fig. 8a (where the cover is of the type "floating cover") the cover 2 is provided with a support plate 25 on the upper side (this plate need not be of titanium but may be of a cheaper construction material). The sealing compound 27 is placed between the support plate 25 and the upper part 26 of the old, residuous rubber insulation. Together with the new prefabricated titanium lining 1, the support plate 25 forms an enclosure of the cover 2, by means of the titanium lining 1 being folded up around the edge of the cover, and joined to a turned up edge of the support plate 25. These are held together by means of a bolt joint 28. In the bolt joint 28 there is also included a clamping rail 29, 30 on each side of the laid-together plates 1, 25. The inner clamping rail 29 is a common flat bar-iron or similar. Between the outer clamping rail 30 and the titanium plate 1 a rubber sealing or fluoroplastic sealing 31 is arranged The outer clamping rail 30 has, furthermore, been given a profile having a "J" cross section. Between the clamping rail 30 and the sealing 31 the cloth 22 of fluoroplastic is placed, which thereby pliably follows the contour of the profiled "J" rail 30. The cloth 22 is secured onto the cell box's edges 24 (cf. Fig. 2 and 3) and forms the flexible part of the cell cover 2 which allows the cover to move vertically. Fig. 8b illustrates an embodiment where one may refrain from a support plate arranged on the upper side. Instead one arranges a sealing having a special profiled sealing element 32, 33, made of titanium. The titanium lining 1, which covers the underside, is the same as that of Fig. 8a. It is folded up around the edge of the cover as in Fig. 8a, and sealing compound is provided in the space between titanium and steel in the cover. On the inside of the folded-up edge of the titanium lining a packing 34 is provided, preferably having a hardness of >120° Shore. PTFE-cloth 22 is folded over the edge of the titanium, whereby a profile 35 having a circular cross section is arranged on the outside of the folded titanium edge at its upper end, so that the PTFE-cloth 22 connects smoothly against the cell cover 2. Inside the folded-up edge of the titanium lining 1 a profiled spring element 33 is laid on the cell cover having a distance element 36 therebetween. The spring element 33 has a cross section of a "J" whose vertical section is somewhat curved in the same direction as the hook of the "J". Over the folded up edge of the lining 1, thus, outside the PTFE-cloth 22, a locking device 32 is arranged, the cross section of which has an "S" form, and whose upper curve is extended by a straight, horizontal part, which ends in a downward/inward curve, to form a hooking-up part. This hooking-up part is brought into engagement with the "J" profiled spring element 33. Pin bolts 16 arranged on the cell cover, L-beam 37 and clamping rail 38 are used to tighten the whole aggregate. The illustrated clamping rail 38 in Fig. 8b has semicircular cross-section, and lies against the spring element 33 and presses it downwards. The pressure force is achieved by the beam, in its turn, being pressed downwards when the nut 17 is screwed down the bolt 16. On the outer edge of the L-beam a locking piece 39 has been welded on. This stops the sealing element 32 from gliding off due to the effect of the spring element 33, which starts to press this upwards simultaneously as the clamping rail 38 clamps fast the spring element 33. Fig. 3 illustrates a third alternative of securely clamping the fluoroplastic in the cell cover 2 and the cell box. In the cell box the cloth is fastened securely in the same manner as before. In the cell cover the cloth 22 is braced by means of the titanium plate, which comprises the lining 1, being produced with an "over dimension" compared to the cell cover 2. Along the entire periphery of the titanium plate the plate is bent or flanged to 180°. In the resulting space a steel beam 40 provided with bin bolts 16 is placed. With an angle bar 41, according to Fig. 3, the cloth 22 may be clamped. A fluoroplastic packing 42, e.g. GORETEX, is laid in the joint. The flanged part of the titanium plate, together with the steel beam may be replaced, by a titanium beam with pin bolts being welded into the titanium plate. It is also possible to laminate the titanium and PTFE so that the cloth fits securely to the metal. For cells having completely flexible cell covers of polymer material the flexible cloth may be replaced by a titanium plate. This is suitably corrugated to give good mechanical stability while maintaining small thickness of the material. At each anode lead through a flexible PTFE-cloth is secured to allow up and down movement of the construction. Although the titanium is very resistant to corrosion because a very stable oxide film is formed on its surface, there may arise local environments where corrosion can arise. At high temperatures (>85° C) and low pH (<3), which is a typical environment in a chlorine cell (e.g. at sealing surfaces), the titanium is not entirely stable when chlorides are present. In the columns the levels of oxidizing mediums are low, which give a reduced environment instead of a normal oxidizing environment, and there is, therefor, great risk of corrosion of the titanium. To protect the titanium against such corrosion it is preferably given an anodic potential. This is achieved in one embodiment (see Fig. 9) by joining the titanium in the lining to a resistance 43, suitably an insulated titanium wire 43 having a dimension adapted for achieving a resistance in the wire of 0.5-2 ohm, and a capacity to withstand a power of about 100 W, whereby the anodic potential is reached by the titanium being connected via said wire to a suitable point having anodic potential. This gives a resistor which is corrosion resistant. Furthermore, the risk of the electrolyte (the brine) being polluted by the components which can reduce the overpotential for hydrogen gas development is greatly reduced. Such a reduction of hydrogen overpotential could lead to very serious accidents. The resistance wire is insulated and the contact points are moulded in to prevent them from oxidizing. The wire may, for example, be wrapped round a plastic rod to form a spiral. This method of anodically protecting titanium constructions in electrochemical cells may be applied in other cell constructions, e.g. membrane cells and diaphragm cells for chlorine production or for chlorate production. To prevent column or slit corrosion, the exposed surfaces may be lined with a protective lining consisting of a mixed dioxide of titanium and some noble metal. Also pure noble metal linings or polymer layers may be considered. Examples of suitable compositions are given below: Noble metal oxide linings: commercially available under the trademark DSA (registered trademark) Nobel metal linings: Pt/Ir-linings which are commercially available. Corrosion does not only effect the cell cover but also the pipes which are used for transporting chemicals (saline, anolyte and chlorine) to and from the cells in, for example, a chlorine factory. These pipes have conventionally been manufactured from fibre glass reinforced polyester (GAP). Erosion and chlorination wears down this material and it must be regularly replaced. Chlorination of the components in the material can form highly chlorinated organic compounds such as dioxines. Many such compounds are, as is well known, highly toxic and/or carcinogenic and therefor must be taken care of at high cost. This problem may be eliminated if the leads are made of titanium. The reason why titanium previously was not used is that the material is electrically conductive, which meant corrosion problems and diminished efficiency.

Claims: A method for protecting cell covers (2) of electrochemical cells, such as chlorine-alkali cells or chlorate cells against corrosion, by providing said cell cover (2) with a prefabricated titanium lining (1), by means of clamping joint means, said lining (1) covering all surfaces of the cell cover (2) which during operation come into contact with process liquids which are corrosive or in any other way chemically aggressive. The method according to claim 1, whereby said cell cover is provided with process lead-throughs (4), comprising the following steps: a) said titanium lining (1) is prefabricated by means of a titanium plate of suitable thickness and having outer dimensions adapted to the cell cover (2) in question, being provided with holes (6) corresponding to the lead-throughs (4) in the cell cover (2); b) titanium pipes (5) having an outer diameter somewhat smaller than the inner diameter of the process lead-throughs (4) are welded over said holes (6); and c) the prefabricated titanium plate (1) is secured in the cell cover (5) by means of said joint in such a way that at least the underside and the edge portions are tightly sealed. The method according to claim 2 or 3, comprising that the cell cover, in situ in an electrochemical plant, is released from, e.g. hard rubber linings before the titanium lining is mounted. The method according to any of the previous claims, wherein the titanium in the lining is protected against corrosion by providing a controlled anodic potential thereto. The method according to claim 4, wherein the anodic potential is achieved by connecting the titanium via an electric resistance (43) to a suitable point having anodic potential, such as by welding onto the lining (1) a titanium wire having a dimension adapted to yield a resistance in the wire of 0.5 - 2 ohm, and a power tolerance of about 100 W. The method according to any of the previous claims, wherein a cloth (22) of e.g. PTFE or other chemically resistant material is secured at the periphery (24) of the cover as an enclosure and for achieving mobility of the cover in vertical direction. The method according to any of the previous claims, wherein the welded-on pipes (5) of the titanium plate are provided with a thread (7), whereby the lining (1) by means of a nut (8) which is screwed onto said thread (7) is secured in the cell cover. The method according to any of claims 1-6, comprising tightly securing the titanium plate secured on the cell cover between a sealing ring (13), which is pressed into the column between the pipes (5), and a protective sleeve (10) for the anodes which are passed through the process lead throughs (4). The method according to any of claims 6-8, comprising fastening said cloth with screws with the aid of a clamping beam. The method according to any of the previous claims, comprising laminating, glueing or welding said cloth to the titanium. The method according to any of the previous claims, comprising protecting the titanium, at least at the sealing areas and columns, with a mixed oxide lining consisting of oxides of titanium and noble metal, or with a lining of pure noble metal. A prefabricated titanium lining (1) for cell covers (2) of electrochemical cells, for the purpose of protecting against corrosion, such as chlorine-alkali cells or chlorate cells, said lining comprising a titanium plate of suitable thickness and having outer dimensions adapted to the cell cover (2) in question, said plate being provided with holes (6) corresponding to the process lead-throughs (4) in the cell cover (2), over which holes (6) pipes (5) are welded, the outer diameter of which being somewhat smaller than the inner diameter of the process lead-throughs (4), whereby the lining (1) is designed to be mounted onto the cell cover, by means of clamping joint means (5, 8, 10, 11; 5, 13, 14, 15; 28, 29, 30, 31). Titanium lining according to claim 12, comprising that the titanium in the lining is protected against corrosion by being given a controlled anodic potential. Titanium lining according to claim 13, provided with an electric resistance having a resistance of 0.5-2 ohm, and an power tolerance of about 100 W, whereby the anodic potential is achieved by connecting the titanium via said resistance to a suitable point having anodic potential. Titanium lining according to claim 14, wherein the resistance comprises a titanium wire having a dimension adapted to yield said characteristics. Titanium lining according to any of claims 12-15, wherein the titanium at least at the sealing areas and columns is protected with a mixed oxide lining consisting of oxides of titanium and noble metal, or having a lining of pure noble metal. An electrochemical cell comprising a cell cover provided with corrosion protection in accordance with any of the preceding claims.

Inventors: BOQUIST SVEN-OLOF, KARLSSON MAGNUS, KROON MARTIN, ULLMAN ANDERS, BOQUIST, SVEN-OLOF, KARLSSON, MAGNUS, KROON, MARTIN, ULLMAN, ANDERS
Applicants: PERMASCAND AB
Application year: 1994
Main classification: C25B9
IPC codes: C25B9